1. Technical Field
The invention relates to image sensor systems. More particularly, the invention relates to a sense amplifier having a precision analog reference level for use with image sensors.
2. Description of the Related Art
Digital photography is one of the most exciting technologies to have emerged during the twentieth century. With the appropriate hardware and software (and a little knowledge), anyone can put the principles of digital photography to work. Digital cameras, for example, are on the cutting edge of digital photography. Recent product introductions, technological advancements, and price cuts, along with the emergence of email and the World Wide Web, have helped make the digital cameras one of the hottest new category of consumer electronics products.
Digital cameras, however, do not work in the same way as traditional film cameras do. In fact, they are more closely related to computer scanners, copiers, or fax machines. Most digital cameras use an image sensor or photosensitive device, such as charged-coupled device (CCD) or complementary metal-oxide semiconductor (CMOS) to sense a scene. The photosensitive device reacts to light reflected from the scene and can translate the strength of that reaction into a numeric equivalent. By passing light through red, green, and blue filters, for example, the reaction can be gauged for each separate color spectrum. When the readings are combined and evaluated via software, the camera can determine the specific color of each element of the picture. Because the image is actually a collection of numeric data, it can easily be downloaded into a computer and manipulated for more artistic effects.
In many digital imaging applications, it is desirable to integrate analog-to-digital conversion (ADC) with an area image sensor, as described in B. Fowler et al, CMOS Image Sensor with Pixel Level A/D Conversion, U.S. Pat. No. 5,461,425. Such integration helps lower system cost, power consumption and improve system performance. Among the different schemes for integrating the ADC with an area image sensor, pixel level ADC promises to achieve the lowest power, and the simplest, most process portable and scalable design. The ADC approach described in U.S. Pat. No. 5,461,425 is based on first order sigma delta modulation which has the advantage of requiring fairly simple and robust circuits. Further with the digital values directly provided by each of the pixel elements, the readout of the digital values could be of extremely high.
Thus, in the image sensor architecture disclosed in U.S. Pat. No. 5,461,425, each pixel or group of pixels contains an ADC so that the readout from the pixel to the sensor array periphery is fully digital. Typically, the readout circuit is a ROM or a single ended SRAM. Such circuit comprises a pixel inside a sensor array and a pixel output bitline. A transistor serves as a buffer readout transistor, and another transistor serves as a select transistor, which is controlled by a signal typically labeled as WORD.
On the other end of the bitline there is a sense amplifier for detecting the digital output of the pixel. Because there are typically many such sense amplifiers required for a large pixel array, it is desirable that the sense amplifiers be high speed, low noise, and power efficient.
In the image sensors designed in the past (see, for example, ISSCC94), the sense amplifiers are single-ended to save area in the pixel, but the bitline swings rail to rail, i.e. Vdd to Gnd (i.e. ground). Such full rail swing generates noise and consumes a considerable amount of power.
It would be advantageous to provide a robust sense amplifier, for example for use with the image sensors, that features high speed, low noise, and power efficiency.
The invention concerns a robust sense amplifier, for example, for use with the image sensors disclosed in U.S. Pat. No. 5,461,425 or preferably in an improved version thereof that features high speed, low noise, and power efficiency. The presently preferred embodiment of the invention comprises a single ended bit-line from each pixel, small swing bit-line detection, a regenerative sense amplifier, and reference generation using precision analog references. In particular, the invention provides a substantial departure from the state of the art, for example, because the use of precision analog references for reference generation is believed unknown in present day sense amplifiers that typically comprise purely digital elements.
Accordingly one of the objects of the present invention is to provide a sense amplifier having a precision analog reference level to be used in images sensors.
Other objects, benefits and advantages together with the foregoing are attained in the exercise of the invention in the following description and resulting in the embodiment illustrated in the accompanying drawings.